There have been proposed several methods to prevent image quality deterioration caused by changes over time or the like for image forming apparatuses, such as an electrophotographic copying machine and a laser printer, and these methods are already put into practice.
The image quality state of the image forming apparatus deteriorates gradually as the apparatus keeps forming images under the same conditions. The image quality deteriorates due to changes over time. More specifically, it occurs when initial image density can be no longer obtained as the charged state of a photosensitive body serving as a recording medium changes with an increase of the number of times of the image forming. Also, the image quality deteriorates when toner particles adhere to the background of the photosensitive body in a region where no toner particles are supposed to.
To eliminate such image quality deterioration in a simple manner, the following method has been proposed. That is, the number of times of the image forming by the image forming apparatus is counted, and a voltage supplied to charging means is controlled in accordance with the counted value, so that a charged potential of the photosensitive body is compensated to a predetermined state.
Also, the above method has been improved as follows. A charged potential of the photosensitive body is measured actually, and a voltage supplied to the charging means is controlled in accordance with the measuring result, so that the charged potential of the photosensitive body is maintained at a constant level to compensate the image quality. According to this improved method, since the charged potential of the photosensitive body can be maintained at a constant level in a secure manner, compensation accuracy is improved compared with the method to control the charged potential of the photosensitive body in accordance with the number of times of the image forming.
According to these methods, the charged potential of the photosensitive body is controlled to stay at a constant level at all times. However, controlling the charged potential of the photosensitive body alone is not enough, because an amount of friction-charge in the toner or the like varies with the environmental changes, such as changes in temperature and humidity, during the actual image forming process. Thus, even if the above methods are adopted, the toner density may differ from the initial density state or unwanted toner particles may adhere to a region in the background when a toner image is actually formed on the photosensitive body.
To solve the above problem, the following method has been proposed and put into practical use to further stabilize the image quality state. In this method, a test patch image (density detecting image) is formed directly on the photosensitive body, and the density of the test patch image is detected. Then, the detecting result and a predetermined reference density value are compared, and the image forming conditions are controlled in accordance with the comparison result, so that a test patch image having a reference image density will be formed. The image forming conditions referred herein are the charged potential of the photosensitive body applied by the charging means, an amount of light emitted from an exposing lamp, a developing bias from a developing device, and the like. The aforementioned unwanted adhesion of the toner particles to a region in the background can be prevented by controlling an amount of exposure, developing bias, etc.
As has been explained, the density of a toner image formed on the photosensitive body varies with not only the deterioration of the photosensitive body, but also the environmental changes, such as changes in temperature and humidity. However, since the above method can handle the environmental changes as well, it can realize more stable image quality compensation.
In case of a color image forming apparatus for forming a color image, when a toner image of each color formed on the photosensitive body is transferred onto a sheet, such as a normal paper serving as a transfer material, the toner image of each color must be superimposed sequentially one on another. Thus, the color image forming apparatus adopts a method using a transfer drum which winds the sheet around and transports the same to a transfer position. An intermediate transfer medium may be provided further, so that a toner image of each color formed on the photosensitive body is transferred thereon sequentially and superimposed one on another, after which the resulting image is transferred onto the sheet.
In the above color image forming apparatus, a testing toner patch image formed on the photosensitive body is transferred onto a transfer drum (including the intermediate transfer medium) touching the photosensitive body, and the density is detected by reading reflected light from the transferred toner patch image. As has been explained, a charging voltage supplied to the charging means is controlled in accordance with the density detection result and the density is adjusted to the reference density to compensate the image quality. According to this method, the density is detected after the toner image formed on the photosensitive body is actually transferred onto the sheet. Thus, it is obvious that better image compensation can be realized.
A method of such image quality compensation is disclosed in, for example, Japanese Laid-open Patent Application No. 11935/1994 (Tokukaihei 6- 11935). In an image forming apparatus disclosed therein, a toner image (test patch image) formed on the photosensitive body with a predetermined number of levels is transferred onto the transfer drum or the like, and the density of the transferred toner image is detected to judge whether the detected density is the reference density or not. A charge potential, a developing bias voltage or the like thereafter is modified in accordance with any difference between the detected density and the reference density so as to adjust the density of the toner image to substantially match the reference density.
Along with the image quality compensation, an image quality stabilizing control is carried out by a developing device which converts an image formed on the photosensitive body, that is, an electrostatic latent image, into a visual toner image. For example, a toner mixing ratio is controlled in addition to the aforementioned developing bias voltage control.
The developing device usually adopts a binary system developing method, and a mixing ratio (toner mixing rate or toner mixing ratio) of the toner and a carrier in a developing bath is maintained at a constant level, so that the toner density of a formed image remains the same. For this reason, the toner mixing ratio is detected by a magnetic permeability sensor and compared with a reference value, and the toner is replenished to the developing bath when the toner mixing ratio is judged to have dropped below the reference value. The density when developing a latent image formed on the photosensitive body is stabilized by controlling the mixing ratio of the toner and carrier in a developing agent to remain at the same level.
Consequently, it has become possible to realize the image quality compensation in response to the changes over time and the environmental changes concurrently by controlling the mixing ratio of the toner and carrier (developing agent) in the developing device to stay at the constant level while carrying out the above image quality stabilizing method at the same time.
On the other hand, Japanese Laid-open Patent Application No. 282166/1994 (Tokukaihei 6-282166) discloses a method for controlling a mixing ratio of the developing agent in the developing device in accordance with the detection result of the changes of the environmental state, particularly, the change in humidity. An image forming apparatus disclosed therein is furnished with means for detecting humidity, especially, absolute humidity. More specifically, the toner is replenished under the control of lowering a mixing ratio target value at normal temperature when the absolute humidity is high, and raising the same when the absolute humidity is low. Thus, the density when developing a latent image formed on the photosensitive body is controlled to remain at the same level in response to the change in humidity, thereby making it possible to stabilize the image quality.
According to the above Japanese Laid-open Patent Application No. 282166/1994 (Tokukaihei 6-282166), a conventional toner density detecting means detects the magnetic permeability of a layer of developing agent adhering to the carrier. Specifically, the quantity of toner particles that adhere to the carrier at any given time depends upon the amount of friction-charge acquired by the toner particles during toner mixing. Hence, the toner density in a layer of developer agent adhering to the carrier at any given point in time may be determined according to the inductance of a coil located near the developing agent layer. Thus, if a predetermined amount of friction-charge is not generated in the toner, an amount of the toner particles adhering to the magnetic carrier varies, which makes it impossible to detect a precise mixing ratio of the developing agent.
Such variance occurs when an amount of the friction-charge in the toner varies while the developing agent is mixed and stirred due to the changes of the environmental conditions, particularly, the change in humidity. In other words, when the humidity is high, an amount of the friction-charge decreases, whereas an amount of the friction-charge increases when the humidity is low, and this causes the aforementioned adverse effect.
To be more specific, in case that the density of the developing agent having a predetermined mixing ratio is detected, a detection signal voltage of the mixing ratio of the developing agent is large (indicating a decrease in an amount of toner) under a high temperature/high relative humidity circumstance (ambient environmental state). On the other hand, the detection signal voltage of the mixing ratio of the developing agent outputted under a high temperature/low relative humidity circumstance or a low temperature/high relative humidity circumstance is similar to the one outputted under a normal temperature/normal relative humidity (indicating a predetermined amount of toner) circumstance. Also, an amount of charge in the toner decreases under a high temperature/high relative humidity circumstance, and more toner particles readily adhere to an electrostatic latent image. Thus, under the above circumstance, if the toner is replenished in a direct response to the detection signal of the mixing ratio of the developing agent, too many toner particles adhere to the electrostatic latent image during the developing process. Therefore, a toner replenish correction control for controlling an amount of replenished toner based on the detection result of humidity or the like is necessary.
In contrast, under the low temperature/high relative humidity and high temperature/low relative humidity circumstances, the detection signal voltage of the mixing ratio of the developing agent is similar to the one outputted under the normal temperature/normal humidity circumstance. Thus, if the toner replenish correction control is carried out based on the above idea with the relative humidity detection signal, the high temperature/high relative humidity circumstance and low temperature/high relative humidity circumstance can not be distinguished from each other, thereby making the corrected state an error. Hence, if the toner replenish correction control is carried out by merely referring to the relative humidity detection signal, the correction control is carried out erroneously under some environmental conditions, and the image density is affected adversely.
As has been explained, since an amount of the charge in the toner varies with humidity, when the magnetic permeability sensor is used as a detector of a mixing ratio of the toner and carrier in the developing bath, an output value from the magnetic permeability sensor varies while the mixing ratio remains the same, thereby causing an incorrect toner mixing ratio control.
To solve the above phenomenon, in case of a method, in which a humidity sensor is provided inside the image forming apparatus and a mixing ratio of the developing agent is corrected to the reference value in accordance with an output from the humidity sensor under the control, the absolute humidity is detected and the mixing ratio of the developing agent is controlled based on the detection result in above-mentioned Japanese Laid-open Patent Application No. 282166/1994 (Tokukaihei 6-282166). In other words, if the relative humidity is used, an erroneous correction may be carried out when the magnetic permeability varies with the environmental changes, which makes the correction unsatisfactory.
However, in case of detecting the absolute humidity, there is another problem of a cost increase, because a temperature sensor is necessary in addition to the humidity sensor. In other words, the sensors for detecting the absolute humidity are very expensive.
On the other hand, in the control device disclosed in above Japanese Laid-open Patent Application No. 11935/1994 (Tokukaihei 6-11935), when the developing bias voltage is varied, for example, a correction response is very quick. However, if a mixing ratio of the developing agent made of the toner and carrier is equal to or smaller than a predetermined value, a desired image density can not be obtained no matter how well the developing bias voltage is controlled.
Thus, another possible correction control is to change a toner mixing ratio of the developing agent along with the control of the developing bias voltage. However, this is not an optimal method, because the responsivity is slowed down if the toner mixing ratio of the developing agent alone is corrected. Moreover, the image density varies considerably with the change of the mixing ratio. Further, in case of above Japanese Laid-open Patent Application No. 282166/1994 (Tokukaihei 6-282166), not only the image density varies with the change in humidity, but also the aforementioned mixing ratio problem occurs. Thus, controlling the mixing ratio based on the relative humidity can make the image quality unstable, thereby making the image quality stabilizing control very difficult.
The reason why the responsivity is slowed down when the mixing ratio of the developing agent is controlled is because it takes a considerable time until the mixing ratio stabilizes since the toner is replenished and mixed with stirring.
On the other hand, in the image quality compensating method disclosed in above Japanese Laid-open Patent Application No. 11935/1994 (Tokukaihei 6-11935), the test patch image is formed merely in accordance with the predetermined levels. However, an amount of reflected light differs significantly depending on the colors of the test patch images, for example, color toner and black toner. Thus, forming exactly the same test patch image for each color is not enough to carry out the correction control in a satisfactory manner.